Active, durable metal nanocatalysts with low platinum (Pt) content are desired for commercializing fuel cells, and for lowering the cost of hydrogen generators through water electrolysis. While several core-shell nanoparticles with a narrow distribution of particle size have displayed high catalytic performance, methods to produce them in large quantity, uniformly and inexpensively are essential for marketing them.
Core-shell nanocatalysts have been made mainly by (1) Pt galvanic replacement of an underpotentially deposited (UPD) Cu monolayers, and (2) forming a Pt-rich shell through high-temperature annealing or acid leaching of Pt-bimetallic alloy particles. To narrow down the spread of particle size, surfactants or capping agents were used during wet-chemical syntheses.
The synthesis of monodispersed noble metal nanoparticles using ethylene glycol as the solvent and reducing agent was reported in 1999. PVP was used as capping agent in synthesizing nanoparticles of Pt, Pd, Au, Ru, and Ir at various temperatures ranging from 100° C. to 150° C.
Multiple processes have been used in preparing catalysts, often involving costly or poisonous agents. For example, a recent study prepared monodispersed Pt-bimetallic alloy nanocatalysts, Pt3M (where M=Fe, Ni, or Co) by an organic solvothermal method in several steps. First, metal salts were dissolved in organic solvents with various surfactants and reduced at elevated temperatures. Next, the metal nanoparticles were separated by centrifuge, washing, and then mixed with carbon black. After drying, the catalysts were heated in an oxygen-rich atmosphere to remove the surfactants. Finally, they further were annealed in a reducing atmosphere to eliminate surface oxides. Hence, simpler procedures employing only inexpensive, environmentally benign agents are highly desirable.